I. Field of tile Invention
This invention relates to a whipstock starter mill assembly for milling a window in a well casing and, in particular, to a starter mill incorporating a bearing nose to indicate completion of the milling process.
II. Description of the Prior Art
Modern drilling operations increasingly rely on deviated or horizontal wellbores to efficiently reach petroleum deposits from a main well. In many instances, the wellbore is drilled along a predetermined curvature in order to increase the reach of the well. Still in other cases, the deviated well is drilled from a main vertical well which, following well-known well completion methods, includes a casing liner to maintain well integrity. In order to form the deviated well, a window is formed in the casing using a milling tool and then continuing the drilling operation through the casing window.
Single trip whipstocks have become widely utilized to run and set the whipstock and thereafter mill the casing without tripping the drill string. The whipstock is run into the well on the milling tool and set in the wellbore by either mechanical or hydraulic means. The whipstock typically includes a packer or anchor with a whip having an angled whipface to direct the milling tool against the casing. Detachably connected to the whipstock is the milling tool which is connected to the drill string. Upon setting of the whipstock at the proper orientation within the casing, the milling tool is disconnected from the whip in order to initiate the milling process. Although the extent of milling can be predetermined many times, it remains a guessing game as to when the casing is sufficiently milled. If too little of the casing is milled away, too small a window is formed in the casing. Conversely, damage to the milling tool and/or the whipstock may occur if the milling tool is forced too far downhole. The milling tool may even become jammed in the wellbore. Thus, it becomes necessary to accurately determine the extent of milling of the casing.
The most widely used method of determining the extent of milling is to monitor the travel of the drill string during the milling operation, although this can prove to be inaccurate due to fluctuations in the drill string and deviations in the wellbore. Tattletales have also been developed which are sheared at a specified point signaling that the milling operation should be stopped. Such tattletales depend upon a variation in hydraulic pressure.